1. Field of the Invention
The present invention relates to memory and applications that use memory, and more particularly to operating applications that use memory and memory having different operating modes with different power requirements, in accordance with available power.
2. Description of Related Art
Traditional channel hot electron ("CHE") Flash memory is used in many applications, but is not entirely suitable for use in portable battery operated applications because of its high active current consumption--on the order of 50 mA--and because of its need for two power supplies, V.sub.CC and V.sub.PP. However, programmable non-volatile memories requiring only a single power supply and having low power consumption, low voltage, and high density characteristics have become available, and have enabled or facilitated a variety of battery-operated applications. For example, the type NX25F080 memory available from Nexcom Technology, Inc. of Sunnyvale, Calif., is an 8 Mbit Serial Flash memory which has an SPI interface and draws only 5 mA of current at a V.sub.CC supply of 3.0 volts. The NX25F080 Serial Flash memory is tailored for battery operated microcontroller-based applications that use small form factor memory to store and/or transfer data, audio and images in a portable or mobile environment. Applications include digital cameras, portable scanners, voice/data recorders and pagers, cellular phones, hand held terminals, and portable data acquisition equipment.
In certain markets such as the consumer electronic market, the user application is extremely cost sensitive. Unfortunately, many battery operated consumer electronic products must include a costly voltage regulator to handle the problem of battery output voltage drop as the battery discharges. For example, an AA or AAA alkaline battery starts fresh with a 1.5 V output voltage and discharges with use until reaching its "end of life" voltage of 0.9 V. Where an electrical device is able to tolerate only a narrow range of voltages, from a low voltage that is high enough to operate the device properly to a high voltage that is low enough not to damage the device, a range typically of about ten percent (i.e., 5 V.+-.10% or 3.3 V.+-.10%), the system designer typically provides an on-board voltage regulator such as the step-up voltage regulator 110 shown in FIG. 1. FIG. 1 shows three batteries 102, 104 and 106, which provide the power input to the voltage regulator 110. The output of the voltage regulator 110 is furnished, for example, to a microcontroller 120, memory 130, and an application 140. If the output of the voltage regulator 110 drops below the low voltage specification, a reset output pin is asserted to notify the microcontroller 120 of a power failure. Unfortunately, this approach adds one or more components to the product, thereby increasing cost and shortening battery life.
Alternatively, a system designer may use components in the device having wider operational voltage ranges. For example, if all the components in a device have an operational range of from 2.7 V (3.times.0.9 V) to 4.5 V (3.times.1.5 V), then three AAA batteries can be used to power the product without the need for an on-board voltage regulator. In this case, more than 90% of the capacity of the batteries can be utilized. However when the supply voltage drops below 2.7 V, the system needs to be notified about the end of life of the batteries. For this, a CPU supervisor typically is utilized. CPU supervisors are available from numerous vendors, including Dallas Semiconductor of Dallas, Tex., which offers a family of such chips under the product numbers DS123.times.. FIG. 2 shows a CPU supervisor 210 installed in a system. If V.sub.CC drops below a certain voltage (in the case of the DS123.times. products, 4.5 V for 5 V systems and 2.7 V for 3.0 V systems), then a Reset output pin is asserted to notify the microcontroller 120 about a power failure so that the microcontroller 120 can act accordingly. Unfortunately, this approach retains a disadvantage of the previous method, which is the need for an additional chip which increases the cost of the product.
In order to address the cost issue associated with adding chips to a memory product to improve its operational voltage range, some memory integrated circuits combine a CPU supervisor with memory, as shown by the memory 330 in FIG. 3. For example, a single chip solution containing 4K bits of EEPROM memory as well as a CPU supervisor and a reset controller circuit is available as product number X25045 from Xicor Semiconductor, Inc. of San Jose, Calif. This chip monitors the system supply voltage level and generates a reset when the voltage declines below a certain value. In the 5 V part (X25043) a reset is asserted when the supply voltage falls below 4.5 V, while in the 3 V part (X25043-2.7) a reset is asserted when the supply voltage falls below 2.7 V.
Although the use of a CPU supervisor in combination with memory achieves a greater operational voltage range without significant additional cost, further improvement would be desirable.